> One issue on Linux is that the zombie process keeps the pid used until 
> the parent reads the child exit status, and Linux pids are limited to
> 32768 by default.

Windows allocates Process and Thread IDs out of a kernel handle table, which can grow to about 2**24 entries (more than 16 million). So the practical resource limit for inactive Process and Thread objects is available memory, not running out of PID/TID values.

> Linux (for example) has the same design: the kernel doesn't keep a 
> "full process" alive, but a lightweight structure just for its parent
> process which gets the exit status. That's the concept of "zombie 
> process".

In Unix, the zombie remains visible in the task list (marked as <defunct> in Linux), but in Windows an exited process is removed from the Process Manager's active list, so it's no longer visible to users. Also, a Process object is reaped as soon as the last reference to it is closed, since clearly no one needs it anymore. 

> The subprocess module uses a magic Handle object which calls 
> CloseHandle(handle) in its __del__() method. I dislike relying on 
> destructors. If an object is kept alive by a reference cycle, it's
> never released: CloseHandle() isn't called.

We could call self._handle.Close() in _wait(), right after calling GetExitCodeProcess(self._handle). With this change, __exit__ will ensure that _handle gets closed in a deterministic context. Code that needs the handle indefinitely can call _handle.Detach() before exiting the with-statement context, but that should rarely be necessary.

I don't understand emitting a resource warning in Popen.__del__ if a process hasn't been waited on until completion beforehand (i.e. self.returncode is None). If a script wants to be strict about this, it can use a with statement, which is documented to wait on the process. 

I do understand emitting a resource warning in Popen.__del__ if self._internal_poll() is None. In this case, in Unix only now, the process gets added to the _active list to try to avoid leaking a zombie. The list gets polled in subprocess._cleanup, which is called in Popen.__init__. Shouldn't _cleanup also be set as an atexit function?

There should be a way to indicate a Popen instance is intended to continue running detached from our process, so scripts don't have to ignore an irrelevant resource warning.